Many types of integrated circuits require a precision current reference. Such a precision current reference is often generated using a voltage-to-current (V-I) converter, examples of which are shown in and described with reference to FIGS. 1A-1C and 2. In these FIGS., examples of circuits that require a precision reference current are shown generically as a load that is labeled Z_load.
In FIG. 1A, the non-inverting (+) input of an amplifier AMP receives a reference voltage Vref. The output of the amplifier AMP drives the gate of an NMOS transistor M1. The inverting (−) input of the amplifier AMP (e.g., an operation amplifier) is connected to the source of the transistor M1. A precision resistor R_precision is connected between the source of the transistor M1 and ground (gnd). A load Z_Load is connected between a supply voltage Vsupply and the drain of the transistor M1. Examples of the load Z_load include, but are not limited to, a resistor, a current mirror input, a reference for a digital-to-analog converter (DAC), a reference for an analog-to-digital converter (ADC), and a capacitor that is used to generate a ramp voltage.
In FIG. 2, the inverting (−) input of an amplifier AMP receives the reference voltage Vref. The output of the amplifier AMP drives the gate of a PMOS transistor M2. The non-inverting (+) input of the amplifier AMP is connected to the drain of the transistor M2. A precision resistor R_precision is connected between the drain of the transistor M2 and ground (gnd). The load Z_Load is connected between the supply voltage Vsupply and the source of the transistor M2.
The circuits of FIGS. 1A and 2 both copy the reference voltage Vref to a precision resistor R_precision using the high gain voltage amplifier AMP in a unity gain buffer configuration. The precision resistor R_precision is used to specify a magnitude of the reference current. The resulting current, which flows through the precision resistor R_precision flows through the pass transistor device, M1 or M2, into the load Z_Load. The circuit in FIG. 2 has advantages for certain applications; however it has poor supply rejection because any perturbation of the supply voltage Vsupply directly modulates the gate-to-source voltage of the transistor M2 and causes a current change in the load Z_Load. Because of this, for applications that require a high power supply rejection ratio (PSRR), the circuit of FIG. 1A is often preferred.
The V-I circuit shown in FIG. 1A makes for an excellent precision current reference, as the only error source is the offset voltage of the amplifier AMP (e.g., an operation amplifier) and the tolerance of the precision external resistor R_precision. However, a real world problem with the circuit of FIG. 1A can be appreciated from FIG. 1B. Referring to FIG. 1B, the AMP, the transistor M1 and the load Z_Load are shown as being within an integrated circuit (IC) package (an IC package can also be referred to as a chip). The precision resistor R_precision is shown as being located outside the IC package on a printed circuit (PC) board, and connected to the IC package (and more specifically, to the source of the transistor M1) by a package pin. Because the precision resistor R_precision is on the PC board and not in the IC package, there is the possibility of an adjacent pin and/or a nearby signal capacitively coupling noise or spurs into the V-I circuit and causing errors. For instance, if there were a high speed comparator comparing the voltage at the drain of the transistor M1 to another signal on the chip, a coupled signal into the package pin could trip the comparator. For another example, if the reference were used as the reference current for an analog-to-digital converter (ADC) or a digital-to-analog converter (DAC), the coupled noise could show up as a degradation in the effective number of bits. In FIG. 1B, the coupled noise is modeled as a voltage source V0, and a parasitic capacitance is modeled as a capacitor C_parasitic. Such a parasitic capacitance can occur, e.g., due to a pin to pin capacitance of the package and/or a trace to trace capacitance on the PC board.
It is also noted that digital signals which swing from the minus supply to the positive supply very rapidly and signals which switch large currents (e.g., the output of a switch mode power supply or a gate driver) can cause a problem when coupled back into the chip.
There is a need to reject the noise coupled into the package pin, which is complicated by the fact that the coupled noise is indistinguishable from a change in the precision resistor R_precision, and the feedback of the amplifier AMP forces all the current injected through the parasitic capacitance to flow into the load Z_load.